Hydrophilization of porous polypropylene membranes by atomic layer deposition of TiO2 for simultaneously improved permeability and selectivity

• Deposition of TiO2 on nascent PP membranes slightly enhanced the hydrophilicity.
• Plasma surface activation promoted the growth of conformal thin layers of TiO2.
• Deposited membranes showed greatly enhanced hydrophilicity and fouling resistance.
• The flux and retention of deposited PP membranes were simultaneously improved.

The uses of porous polypropylene (PP) membranes are limited in water-based separations because of their strong hydrophobicity. To improve the separation performances of polypropylene membranes by hydrophilization, we deposited TiO2 on their pore surface using the atomic layer deposition strategy with and without a pretreatment to the membrane by plasma. The direct deposition without plasma pretreatment led to slightly enhanced hydrophilicity because TiO2 was slowly deposited on the membrane as discrete particles due to the lack of active groups on the bare polypropylene surface. In contrast, after a short exposure to plasma generated in air, oxygen-containing active groups were formed on the membrane and subsequent atomic layer deposition yielded conformal thin layer of TiO2 on the pore walls. The deposited membranes showed remarkably enhanced hydrophilicity and higher deposition cycles led to stronger hydrophilicity. As a consequence of the enhanced hydrophilicity, the permeability and retention of the deposited membrane were simultaneously improved with big amplitudes. For example, an increase in pure water flux of ~60% and a more than doubled retention ratio were obtained by deposition of TiO2 for 150 cycles on the plasma-activated polypropylene membrane. Furthermore, the TiO2-deposited membranes showed improved resistance to protein fouling compared to unmodified membranes also because of the enhanced hydrophilicity. Such a hydrophilization strategy of plasma pretreatment and subsequent atomic layer deposition of metal oxides is expected to be also effective in the upgrading of performances of other membranes with inert surfaces.